Britain's House of Commons voted decisively to authorise Prime Minister Theresa May to trigger the start of the country's exit from the European Union.

The outcome of the vote was never in doubt, even as MPs spent a second consecutive day arguing the merits of a departure that the bitterly divided country approved in a June referendum.

The margin of the roll call, 498 to 114, gives May a convincing mandate as she prepares to launch divorce talks with the EU by the end of next month.

Once that is done, Britain will have two years to negotiate the terms of its departure.

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The vote was necessitated by a British Supreme Court ruling last week that Parliament, not the Prime Minister, should have the final say on whether Britain leaves the EU.

May's Government had vigorously contested that notion, pursuing appeals in a bid to keep the departure, known as Brexit, from becoming entangled in parliamentary debate.

Her reluctance stemmed from simple arithmetic: Although the British public voted 52 per cent to 48 per cent to quit the EU, most members of Parliament had favoured staying in.

Even so, many pro-remain MPs calculated that the political cost of blocking Brexit would be high, and they chose to align themselves with the public's will.

May had the resounding support of her ruling Conservative Party, which has been divided over Britain's EU membership for decades.

She also won backing from Opposition Leader Jeremy Corbyn and his Labour Party, though a significant number of Labour members bucked their leadership by voting no.

"Those of us who campaigned for remain know that Brexit is to happen," said Stella Creasy, an MP who was among the Labour rebels.

Voting no, she said, was "the only chance to send the Prime Minister back to the drawing board."

The Scottish National Party - the third-largest in the House of Commons - and the Liberal Democrats also lined up against the legislation.

But they came nowhere near stopping the bill, and amendment proposals intended to influence May's position in the exit talks also fell short.

The bill was written as simply as possible to minimise debate and maximise May's latitude for negotiation. In a mere two clauses, it gives May permission to trigger Article 50, the never-before-used mechanism for leaving the EU.

The public "voted leave because they wanted to leave," said Conservative MP David Warburton, urging his colleagues to back the vote.

The bill still needs approval from the House of Lords, but that is considered a formality.

Despite the lack of suspense, MPs staged a passionate debate over some 16 hours, with more than 150 members weighing in.

May has signalled she intends to push for a clean break from the EU, with Britain leaving behind the common European market for goods and services as well as the customs union that regulates members' trade within and outside the bloc.

The PrimeMminister has insisted that Britain intends to transform its ties to Europe, not sever them.

But European leaders have taken a hard line, saying that Britain will not be able to cherry-pick the best parts of EU membership while shunning the responsibilities.

May has also annoyed European allies by seeming to cosy up to US President Donald Trump.

While other European leaders took a cautious approach to a leader seen by many on the continent as erratic and politically toxic, May flew to Washington within a week of Trump's inauguration and proclaimed her desire to strike a trade deal with the new administration.